Intraflagellar transport (IFT) proteins are microtubule based transport machinery, which are essential for the assembly and maintenance of all cilia and flagella. Recent findings have not only revealed the various roles of cilia and flagella in motility, sensory reception, and signaling, but also demonstrated the function of IFT in the control of gene regulation and expression, cell proliferation and differentiation, and animal development and behavior. The mutation of IFT proteins causes the loss or severe reduction of cilia and flagella in various organisms, which can lead to numerous human diseases characterized by various combinations of pathological changes of kidney, retina, and skeleton. Conditional ablation of IFT88 disrupts hedgehog signaling, with polydactyly and defects of endochondral bone formation. Disruption of Kif3a, a subunit of kinesin II, showed limb and cranial skeletal abnormalities. Recently, a novel protein mutated in chondroectodermal dysplasia Ellis-van Creveld syndrome (EVC) was found to be localized to the base of the cilia, and disruption of this gene in mice results in a variety of skeletal and craniofacial abnormalities as well as alterations in the teeth and nails. Mice with targeted deletion of IFT/cilia have a wide variety of bone phenotypes that provide interesting insights into the function of individual IFT protein in bone formation. Most recently, Beales et al group found that the partial loss of IFT80 in human, a novel component of the IFT complex B, causes human diseases such as Jeune asphyxiating thoracic dystrophy (JATD) and short rib polydactyly (SRP) type III. Both diseases have severe bone abnormalities including shortening of the long bones and constriction of the thoracic cage. However, it is unclear how IFT80 mutation leads to skeletal abnormalities. Our recent studies have shown that IFT80 is highly expressed during osteoblast differentiation. Silencing IFT80 not only impairs cilia formation, but also significantly inhibits osteoblast differentiation through down-regulating the expression of osteoblast marker genes- Runx2 and osteocalcin, and hedgehog signaling (Hh) related genes -shh, Ihh and Gli2. These are important results; nonetheless, there is still no in vivo evidence for a general requirement for IFT80 signaling in osteoblast differentiation and bone formation. Most recently, we have generated IFT80 conditional knockout model in osteoblast specific lineage and found that IFT80 mutant mice showed apparent growth retardation with severe bone abnormalities. Based on these results, we hypothesize that IFT80 plays an essential role in vertebrate bone formation and normal bone function through regulating osteoblast gene expression, differentiation and Hh/Gli pathway. In this proposal, we will test the hypothesis by generating an IFT80 conditional knockout allele to study the role of IFT80 in bone formation and investigating the effect of deletion of this gene on osteoblast gene expression, differentiation and proliferation. The long term objective of this work is to understand the functionmechanism and interactions of IFT/cilia proteins in bone development and bone diseases.